Generally speaking, water-based microemulsions containing perfumes or flavors, or yet other hydrophobic active materials, have already been reported in the prior art. Frequently, these prior known microemulsions, in order to provide proper dispersion of the oil in the water-based medium, contain large amounts of short chain alcohols, namely ethanol, or other VOC (Volatile Organic Compound) materials. In the flavor and fragrance field, however, there are advantages in resorting to the use of alternatives to these conventional microemulsions, in particular alternatives with low VOC content, which, although they dispense with the use of significant amounts of the latter, still retain all of the microemulsion's desirable aspects, such as sensorial performance, sprayability, diffusity, and skin feel, commonly associated with the presence of such short chain alcohols and in particular with ethanol.
By “VOC” we mean here the Volatile Organic Compounds as defined by the Environmental Protection Agency, and in particular we mean C1-C5 alkanols, such as ethanol, or C1-C5 alkanediols, such as ethylene glycol.
Low VOC microemulsions have also been reported in the prior art. However, generally, in formulating such microemulsions it is important to increase the total amount of surfactants so as to obviate the absence of VOCs, otherwise the final emulsions display a lack of clarity and/or stability problems, and this is unacceptable for the fragrance and flavor applications. The increase in the amount of surfactants in the final microemulsion typically results in products containing surfactant or tensoactive systems that are often in large excess with respect to the solubilized oil, namely the perfume or flavor. Obviously, a large excess of surfactant is also a disadvantage for such final products, in particular for perfumes or other products intended for application to the skin, hair or other surfaces such as textiles, wherein high surfactant content can lead to foamy, sticky, irritating or allergenic products that are unacceptable to consumers.
In this context, U.S. Pat. No. 5,374,614 provides a clear microemulsion for perfumery applications with a low VOC content and wherein the amount of surfactant used is reduced with regard to previously known similar microemulsions. The products there-described comprise less than 10% w/w of lower aliphatic alcohols and resort to the use of a surfactant mixture comprising at least 50% by weight of a primary non-ionic surfactant, relative to the total weight of surfactant mixture, together with a lower amount of ionic co-surfactant. Although these prior known microemulsions made it possible to provide products having a low content in aliphatic alcohols, they required careful selection of the surfactant system nature and concentration, as a function of the nature and concentration of the fragrance oil to be solubilized therein, to provide the desired clear microemulsion. As is described on column 6, lines 31 to 38 of this document, the perfume oil needed to be tested against a range of surfactants to allow selection of the surfactant possessing the right HLB value for each specific fragrance. In addition, we have been able to establish that, for certain types of fragrances, it was in fact impossible to obtain clear o/w microemulsions based on the teachings of U.S. Pat. No. 5,374,614. This is particularly true for fragrances containing large amounts of terpenes or musks for example. This is shown in the comparative examples presented further on.
Now, it is highly desirable in the perfume and flavor industry to be able to formulate clear o/w flavor and fragrance microemulsions of any flavor or fragrance composition, over large domains of concentrations, without having to significantly change the surfactant system. Moreover, it would be desirable to have available an unselective, or universal, solubilizing system, i.e., that, for any particular surfactant system and specific amount thereof, shows no significant dependency on the nature and concentration of the perfume in order to provide a microemulsion that remains clear and stable under storage conditions.
Moreover, as it follows from the above, it is also highly desirable to provide clear microemulsions that contain low amounts of surfactants and in particular surfactant amounts that are lower than the amount of solubilized oil.
The present invention provides unexpected and highly advantageous solutions to both these requirements.
The aim of the present invention is precisely to provide o/w microemulsions that, for any surfactant or surface active system, remain clear and stable upon concentration or dilution of the oil phase, in other words throughout the whole range of oil concentrations in the microemulsion that are useful in application. The invention therefore is not only an important improvement over the o/w microemulsions disclosed in U.S. Pat. No. 5,374,614, but is essentially universal, effective for any combination of oil and surfactant system that one desires to apply.
The presently described solution results from an unexpected interfacial synergistic effect between the surfactant system and a particular type of chemicals capable of enhancing the ability of any such surfactant system to solubilize any oil, namely perfume or flavor compositions, independently of the nature or concentration of the latter, and thus provide perfectly clear and stable microemulsions thereof, upon dilution in water.